1. Field of the Invention
The present invention relates to communication networks, and, more particularly, to a frame format for selective acknowledgment in a wireless communication network.
2. Description of the Related Art
Wireless local area networks (WLANs) include one or more non-fixed stations (or mobile terminals) such as cell phones, notebook (laptop) computers, and hand-held computers, equipped with generally available, WLAN PC cards that enable them to communicate among themselves as well as through a network server. The network server provides support for communication between mobile terminals in different service sets (service areas) which are associated with different access points (APs). An AP is a terminal or other device that provides connectivity to other networks or service areas, and may be either fixed or mobile. Such WLAN networks allow mobile terminals to be moved within a particular service area without regard to physical connections among the mobile terminals within that service area. An example of a WLAN network is a network that conforms to standards developed and proposed by the Institute of Electrical and Electronic Engineers (IEEE) 802.11 Committee (termed herein as a network operating in accordance with one or more editions of the IEEE 802.11 standard). Typically, all messages transmitted among the mobile terminals of the same cell (i.e., those terminals associated with the same AP) in such WLAN networks are transmitted to the access point (AP) rather than being directly transmitted between the mobile terminals. Such centralized wireless communication provides significant advantages in terms of simplicity of the communication link as well as in power savings.
Most networks are organized as a series of layers, each one built upon its predecessor. The purpose of each layer is to offer services to the higher layers, shielding those layers from implementation details of lower layers. Between each pair of adjacent layers is an interface that defines those services. The International Standards Organization has developed a layered network architecture called the Open Systems Interconnection (OSI) Reference model that has seven protocol layers: application, presentation, session, transport, network, data link, and physical. The function of the lowest level, the physical layer, is to transfer bits over a communication medium. The function of the data link layer is to partition input data into data frames and transmit the frames over the physical layer sequentially. Each data frame includes a header that contains control and sequence information for the frames.
The interface between the data link layer and the physical layer includes a medium access control (MAC) device and a physical layer signaling control device, called a PHY device. The purpose of the MAC device and the PHY device is to ensure two network stations are communicating with the correct frame format and protocol. Not all communication networks require all layers of the OSI model. For example, in a wireless communication networks, physical, network, and application layers are typically sufficient to enable operation.
In WLANs, the physical device is a radio and the physical communication medium is free space. The IEEE 802.11 standard for WLANs defines the communication protocol between a MAC device and a PHY device. According to the WLAN data communication protocol, each data frame transferred between the MAC device and the PHY device has a PHY header, a MAC header, MAC data, and error checking fields. The PHY header includes a preamble that is used to indicate the presence of a signal, unique words, frame length, etc. The MAC header includes frame control, duration, source (i.e., MAC) and destination address, and data sequence number. FIG. 1 shows a typical frame format 100 for 802.11 wireless LAN systems. Frame format 100 comprises packet header 101, payload data 102 and frame check sequence (FCS) 103.
The maximum achievable throughput of 802.11 wireless LAN systems largely depends on the length of the frames that carry the data information (payload). With relatively good channel quality, the throughput efficiency increases when a larger frame size is used. This increase in throughput efficiency is mainly due to the large, fixed-size of the MAC overhead, such as: Inter Frame Space (IFS), preamble and Physical Layer Convergence Protocol (PLCP) header, MAC header, and FCS.
Section 7.6 of the 802.11 version “e” draft D3.0 specifies MAC-Level Forward Error Correction (FEC) and FEC frame formats in which frames are also divided in blocks. The MAC header and each block have their own FEC fields enabling detection and correction of errors in the separate blocks. However, the frame is dropped if one or more of the blocks are corrupted.